The invention relates to a method for transmitting signals in a radio communication system, in particular in a mobile telephone system.
Future radio communication systems will support very high data rates in order to be able to operate multimedia applications with the necessary quality of service. A continued increase in subscriber numbers is also to be anticipated such that further frequency bandwidths must be developed for use by radio communication systems. In order to make efficient use of these frequency bandwidths however radio communication systems need to operate over a large frequency range.
Various methods are used in radio communication systems for segmenting resources and multiplexing. In addition to Time Division Multiplexing (TDM) and Code Division Multiplexing (CDM), various frequency channels are created through Frequency Division Multiplexing (FDM). The FDM method involves segmenting a wide frequency spectrum into many separate frequency channels, each with a narrow bandwidth, within the frequency range, thus creating a frequency channel grid defined by the gaps between the carrier frequencies. Advantageously this allows a plurality of subscribers on different frequency channels to be served simultaneously and resources to be better tailored to individual subscriber requirements. A sufficient distance between frequency channels ensures that interference between the channels can be reduced and controlled.
In order for the narrow band frequency channels to be used by a corresponding access method known as FDMA (Frequency Division Multiple Access), the transmitter and receiver must each select a corresponding carrier frequency in a coordinated manner. Before using the corresponding resource, i.e. the radio channel, continued checks need to be made to establish whether the selected resource is not already being used by other stations. The resource is then reserved, if necessary the reservation being communicated to other potential stations so that these stations do not subsequently access the resource at the same time and cause collisions. The challenge here is to optimize the efficient use of these frequencies, using few resources and as far as possible only one transmitter and receiver (transceiver) for each terminal. A further constraint is that all stations have equal rights i.e. no station assumes a control function for assigning the frequency channels via a plurality of stations. In particular in self organizing networks and networks without an infrastructure, so-called ad-hoc networks, stations with equal priority frequently perform the same algorithms and protocols. A known example for such networks is the local wireless network (Wireless LAN) according to the IEEE 802.11.standard.
If stations in networks such as these are not informed about the use of frequency channels because the appropriate information is not collated and distributed via a central station, a station will be unable to decide which frequency channel it should propose or select for communicating with another station. Also, a station will not know when another station is ready to receive and on which frequency. Consequently, a station would generally be unable to establish a connection with another station if both were free to select the frequency channels. Also, a distribution service (broadcast) on one frequency would only reach one part of the station which happened to be ready to receive on this frequency.
The requirement is therefore to coordinate the use of these orthogonal resources. If a particular frequency is to be used for this purpose, at fixed, predetermined times i.e. all stations are ready to receive on this frequency, then the frequencies which exist alongside them cannot be used. These resources would then be unused and unavailable for these stations.
In existing cellular mobile telephone systems, the frequencies are allocated to the mobile stations (MS) located within one radio cell of the base station via a central station, the base station (BS). With GSM (Global System for Mobile Communication) for example, one central frequency channel per radio cell is used for transmitting general information used by the mobile stations for determining the frequency channel for logon and request of resources. If a mobile station wishes to transmit data, it requests the data from the base station on the frequency channel it knows. The base station then notifies the mobile station of the appropriate carrier frequency on which it can communicate with the base station. The allocation and management of available resources is controlled centrally in a Base Station Controller (BSC) subordinating the base station and signaled from the base station. The same applies to third generation UMTS (Universal Mobile Telecommunication System) systems which also use a base station to signal the frequency allocation according to this central principle.
Because of the central control, this approach cannot be used in a decentrally organized system without a central entity. Other systems, such as for example WLAN systems (Wireless Local Area Networks) according to the HIPERLAN Type 2 Standard known for example from the ETSI/BRAN document “Broadband Radio Access Networks (BRAN); HIPERLAN Type 2 Functional Specifications Data Link Control (DLC) Layer; Part 4—Extension for Home Environments., draft DTR/BRAN-0020004-4, ETSI, Sophia Antipolis, France, April 2000 or according to the IEEE 802.11 standard, use only one carrier frequency for communicating. Switching to a different frequency serves to avoid interference. If a new, usable carrier frequency is found, all stations transmit and receive on this frequency. In HIPERLAN/2 this method is known as Dynamic Frequency Selection (DFS). A simultaneous needs-based use of several frequencies by any stations to increase the total possible capacity of the system or of an individual connection is however not provided. Consequently, the maximum date rate is limited to one frequency channel.
Similar to the DFS method according to the HIPERLAN/2 Standard, a system is proposed in the document by R. Sakata, K. Naha, H. Murata, S. Yoshida Performance Evaluation of Autonomous Decentralized Vehicle-grouping Protocol for Vehicle-to vehicle Communications, in Prov. IEEE VTC, Boston, Mass., Sep. 24-28, 2000 pp 153-157, in which vehicles form groups using different frequencies. In this method, neighboring vehicles share the same carrier frequency. By measuring the active frequency channels, it is possible to participate simultaneously in a plurality of groups and switch groups to take account of the changing network topologies. This assumes that, generally speaking, only one frequency is used for exchanging data whereas the other frequencies only receive so as to prepare for occupancy and a possible change of frequency. In order to be able to receive simultaneously on other frequencies, it is proposed that two transceivers be used, one for data exchange and the second for measuring other potential frequencies. The use of a plurality of available frequencies for communicating with neighboring stations for the exchange of data is however not described, even here.
The DECT cordless telephone standard (Digital Enhanced Cordless Telephone) specifies a flexible use of resources including different frequency channels. The method by which the resources are used is called Dynamic Channel Selection (DCS). Although in this system, when in so-called “Basic Mode”, one base station has exclusive control over resource allocation, the relaying of data packets is supported. To achieve this, as in a distributed, decentralized system, resource availability is checked and occupied.
For each carrier frequency, DECT specifies a frame with 24 time slots, 12 time slots being used for the downlink and 12 for the uplink with a fixed allocation in the Time Division Duplex method (TDD), see FIG. 1.
DECT is thus an FDMA/TDMA/TDD system in which 10 carrier frequencies can generate up to 120 communication (bearer) channels. Before setting up a connection, a station measures different communication channels. The resulting Received Signal Strength Indicator (RSSI) is recorded in a table. All channels with a signal level lower than the lowest level (−93 dBm) are classified as “quiet” and may be used to set up a communication channel. The upper threshold, which is classified as “busy” defines the range of busy channels and is variable. As a rule the upper threshold is set to −33 dBm. Channels with signal levels in excess of this threshold may not be used for setting up a communication channel.
The maximum number of channels in a cell depends on the number of base station transceivers. If only one transceiver is available per base station, only one mobile station per time unit (time slot) can be operated. A free allocation of time slots for direct communication between mobile stations in parallel with the communication between a mobile station and base station is not defined in greater detail. DECT, however, supports this “walkie-talkie” mode. As soon as a mobile station with only one transceiver occupies a channel, the remaining frequency channels parallel to this time slot are marked as so-called blind slots, see FIG. 1. Stations may neither transmit nor receive on these blind slots and conduct measurements of the received signal level. Depending on how quickly the stations are able to switch from transmitting to receiving and vice versa (Transceiver Turn-Around), time slots before and after are also to be marked as blind slots. The base station periodically transmits control information to all mobile stations via the currently busy time slots. This information allows the mobile stations to measure the remaining unoccupied time slots which can be used as potential candidates for a future communication between the base station and mobile station. Owing to the base station's special role, the methods described and implemented in the DECT system are not applicable to a target system in which no central entity exists but in which simultaneous use of time slots on different frequencies is to be facilitated.